The invention relates generally to aluminum containing iron-base alloys useful as electrical resistance heating elements.
Iron base alloys containing aluminum can have ordered and disordered body centered crystal structures. For instance, iron aluminide alloys having intermetallic alloy compositions contain iron and aluminum in various atomic proportions such as Fe3Al, FeAl, FeAl2, FeAl3, and Fe2Al5. Fe3Al intermetallic iron aluminides having a body centered cubic ordered crystal structure are disclosed in U.S. Pat. Nos. 5,320,802; 5,158,744; 5,024,109; and 4,961,903. Such ordered crystal structures generally contain 25 to 40 atomic % Al and alloying additions such as Zr, B, Mo, C, Cr, V, Nb, Si and Y.
An iron alu.rninide alloy having a disordered body centered crystal structure is disclosed in U.S. Pat. No. 5,238,645 wherein the alloy includes, in weight %, 8-9.5 Al, xe2x89xa67 Cr. xe2x89xa64 Mo, xe2x89xa60.05 C, xe2x89xa60.5 Zr and xe2x89xa60.1 Y, preferably 4.5-5.5 Cr, 1.8-2.2 Mo, 0.02-0.032 C and 0.15-0.25 Zr. Except for three binary alloys having 8.46, 12.04 and 15.90 wt % Al, respectively, all of the specific alloy compositions disclosed in the ""645 patent include a minimum of 5 wt % Cr. Further, the ""645 patent states that the alloying elements improve strength, room-temperature ductility, high temperature oxidation resistance, aqueous corrosion resistance and resistance to pitting. The ""645 patent does not relate to electrical resistance heating elements and does not address properties such as thermal fatigue resistance, electrical resistivity or high temperature sag resistance.
Iron-base alloys containing 3-18 wt % Al, 0.05-0.5 wt % Zr, 0.01-0.1 wt % B and optional Cr, Ti and Mo are disclosed in U.S. Pat. No. 3,026,197 and Canadian Patent No. 648,140. The Zr and B are stated to provide grain refinement, the preferred Al content is 10-18 wt % and the alloys are disclosed as having oxidation resistance and workability. However, like the ""645 patent, the ""197 and Canadian patents do not relate to electrical resistance heating elements and do not address properties such as thermal fatigue resistance, electrical resistivity or high temperature sag resistance.
U.S. Pat. No. 3,676,109 discloses an iron-base alloy containing 3-10 wt % Al, 4-8 wt % Cr, about 0.5 wt % Cu, less than 0.05 wt % C, 0.5-2 wt % Ti and optional Mn and B. The ""109 patent discloses that the Cu improves resistance to rust spotting, the Cr avoids embrittlement and the Ti provides precipitation hardening. The ""109 patent states that the alloys are useful for chemical processing equipment. All of the specific examples disclosed in the ""109 patent include 0.5 wt % Cu and at least 1 wt % Cr, with the preferred alloys having at least 9 wt % total Al and Cr, a minimum Cr or Al of at least 6 wt % and a difference between the Al and Cr contents of less than 6 wt %. However, like the ""645 patent, the ""109 patent does not relate to electrical resistance heating elements and does not address properties such as thermal fatigue resistance, electrical resistivity or high temperature sag resistance.
Iron-base aluminum containing alloys for use as electrical resistance heating elements are disclosed in U.S. Pat. Nos. 1,550,508; 1,990,650; and 2,768,915 and in Canadian Patent No. 648,141. The alloys disclosed in the ""508 patent include 20 wt % Al, 10 wt % Mn; 12-15 wt % Al, 6-8 wt % Mn; or 12-16 wt % Al, 2-10 wt % Cr. All of the specific examples disclosed in the ""508 patent include at least 6 wt % Cr and at least 10 wt % Al. The alloys disclosed in the ""650 patent include 16-20 wt %. Al, 5-10 wt % Cr. xe2x89xa60.05 wt % C, xe2x89xa60.25 wt % Si, 0.1-0.5 wt % Ti, xe2x89xa61.5 wt % Mo and 0.4-1.5 wt % Mn and the only specific example includes 17.5 wt % Al, 8.5 wt % Cr, 0.44 wt % Mn, 0.36 wt % Ti, 0.02 wt % C and 0.13 wt % Si. The alloys disclosed in the ""915 patent include 10-18 wt % Al, 1-5 wt % Mo, Ti, Ta, V, Cb, Cr, Ni, B and W and the only specific example includes 16 wt % Al and 3 wt % Mo. The alloys disclosed in the Canadian patent include 6-11 wt % Al, 3-10 wt % Cr. xe2x89xa64 wt % Mn, xe2x89xa61 wt % Si, xe2x89xa60.4 wt % Ti, xe2x89xa60.5 wt % C, 0.2-0.5 wt % Zr and 0.05-0.1 wt % B and the only specific examples include at least 5 wt % Cr.
Resistance heaters of various materials are disclosed in U.S. Pat. No. 5,249,586 and in U.S. patent application Ser. Nos. 07/943,504, 08/118,665, 08/105,346 and 08/224,848.
U.S. Pat. No. 4,334,923 discloses a cold-rollable oxidation resistant iron-base alloy useful for catalytic converters containing xe2x89xa60.05% C, 0.1-2% Si, 2-8% Al, 0.02-1% Y,  less than 0.009% P,  less than 0.006% S and  less than 0.009% O.
U.S. Pat. No. 4,684,505 discloses a heat resistant iron-base alloy containing 10-22.7% Al, 2-12% Ti, 2-12% Mo, 0.1-1.2% Hf, xe2x89xa61.5% Si, xe2x89xa60.3.T% C, xe2x89xa60.2% B, xe2x89xa61.0% Ta, xe2x89xa60.5% W, xe2x89xa60.5% V, xe2x89xa60.5% Mn, xe2x89xa60.3% Co, xe2x89xa60.3% Nb, and xe2x89xa60.2% La. The ""505 patent discloses a specific alloy having 16% Al, 0.5% Hf, 4% Mo, 3% Si, 4% Ti and 0.2% C.
Japanese Laid-open Patent Application No. 53-119721 discloses a wear resistant, high magnetic permeability alloy having good workability and containing 1.5-17% Al, 0.2-15% Cr and 0.01-8% total of optional additions of  less than 4% Si,  less than 8% Mo,  less than 8% W,  less than 8% Ti,  less than 8% Ge,  less than 8% Cu,  less than 8% V,  less than 8% Mn,  less than 8% Nb,  less than 8% Ta,  less than 8% Ni,  less than 8% Co,  less than 3% Sn,  less than 3% Sb,  less than 3% Be,  less than 3% Hf,  less than 3% Zr,  less than 0.5% Pb, and  less than 3% rare earth metal. Except for a 16% Al, balance Fe alloy, all of the specific examples in Japan ""721 include at least 1% Cr and except for a 5% Al, 3% Cr, balance Fe alloy, the remaining examples in Japan ""721 include xe2x89xa710% Al.
A 1990 publication in Advances in Powder Metallurgy, Vol. 2, by J. R. Knibloe et al., entitled xe2x80x9cMicrostructure And Mechanical Properties of P/M Fe3Al Alloysxe2x80x9d, pp. 219-231, discloses a powder metallurgical process for preparing Fe3Al containing 2 and 5% Cr by using an inert gas atomizer. This publication explains that Fe3Al alloys have a DO3 structure at low temperatures and transform to a B2 structure above about 550xc2x0 C. To make sheet, the powders were canned in mild steel, evacuated and hot extruded at 1000xc2x0 C. to an area reduction ratio of 9:1. After removing from the steel can, the alloy extrusion was hot forged at 1000xc2x0 C. to 0.340 inch thick, rolled at 800xc2x0 C. to sheet approximately 0.10 inch thick and fish rolled at 650xc2x0 C. to 0.030 inch. According to this publication, the atomized powders were generally spherical and provided dense extrusions and room temperature ductility approaching 20% was achieved by maximizing the amount of B2 structure.
A 1991 publication in Mat. Res. Soc. Symp. Proc., Vol. 213, by V. K. Sikka entitled xe2x80x9cPowder Processing of Fe3Al-Based Iron-Aluminide Alloys,xe2x80x9d pp. 901-906, discloses a process of preparing 2 and 5% Cr containing Fe3Al-based iron-aluminide powders fabricated into sheet. This publication states that the powders were prepared by nitrogen-gas atomization and argon-gas atomization. The nitrogen-gas atomized powders had low levels of oxygen (130 ppm) and nitrogen (30 ppm). To make sheet, the powders were canned in mild steel and hot extruded at 1000xc2x0 C. to an area reduction ratio of 9:1. The extruded nitrogen-gas atomized powder had a grain size of 30 xcexcm. The steel can was removed and the bars were forged 50% at 1000xc2x0 C., rolled 50% at 850xc2x0 C. and finish rolled 50% at 650xc2x0 C. to 0.76 mm sheet.
A paper by V. K. Sikka et al., entitled xe2x80x9cPowder Production, Processing, and Properties of Fe3Alxe2x80x9d, pp. 1-11, presented at the 1990 Powder Metallurgy Conference Exhibition in Pittsburgh, Pa., discloses a process of preparing Fe3Al powder by melting constituent metals under a protective atmosphere, passing the metal through a metering nozzle and disintegrating the melt by impingement of the melt stream with nitrogen atomizing gas. The powder had low oxygen (130 ppm) and nitrogen (30 ppm) and was spherical. An extruded bar was produced by filling a 76 mm mild steel can with the powder, evacuating the can, heating 1xc2xd hr at 1000xc2x0 C. and extruding the can through a 25 mm die for a 9:1 reduction. The grain size of the extruded bar was 20 xcexcm. A sheet 0.76 mm thick was produced by removing the can, forging 50% at 1000xc2x0 C., rolling 50% at 850xc2x0 C. and finish rolling 50% at 650xc2x0 C.
Oxide dispersion strengthened iron-base alloy powders are disclosed in U.S. Pat. Nos. 4,391,634 and 5,032,190. The ""634 patent discloses Ti-free alloys containing 10-40% Cr, 1-10% Al and xe2x89xa610% oxide dispersoid. The ""190 patent discloses a method of forming sheet from alloy MA 956 having 75% Fe, 20% Cr, 4.5% Al, 0.5% Ti and 0.5% Y2O3.
A publication by A. LeFort et al., entitled xe2x80x9cMechanical Behavior of FeAl40 Intermetallic Alloysxe2x80x9d presented at the Proceedings of International Symposium on Intermetallic Compoundsxe2x80x94Structure and Mechanical Properties (JIMIS-6), pp. 579-583, held in Sendai, Japan on Jun. 17-20, 1991, discloses various properties of FeAl alloys (25 wt % Al) with additions of boron, zirconium, chromium and cerium. The alloys were prepared by vacuum casting and extruding at 1100xc2x0 C. or formed by compression at 1000xc2x0 C. and 1100xc2x0 C. This article explains that the excellent resistance of FeAl compounds in oxidizing and sulfidizing conditions is due to the high Al content and the stability of the B2 ordered structure.
A publication by D. Pocci et al., entitled xe2x80x9cProduction and Properties of CSM FeAl Intermetallic Alloysxe2x80x9d presented at the Minerals, Metals and Materials Society Conference (1994 TMS Conference) on xe2x80x9cProcessing, Properties and Applications of Iron 1.6 Aluminidesxe2x80x9d, pp. 19-30, held in San Francisco, Calif. on Feb. 27-Mar. 3, 1994, discloses various properties of Fe40Al intermetallic compounds processed by different techniques such as casting and extrusion, gas atomization of powder and extrusion and mechanical alloying of powder and extrusion and that mechanical alloying has been employed to reinforce the material with a fine oxide dispersion. The article states that FeAl alloys were prepared having a B2 ordered crystal structure, an Al content ranging from 23 to 25 wt % (about 40 at %) and alloying additions of Zr, Cr, Ce, C, B and Y2O3. The article states that the materials are candidates as structural materials in corrosive environments at high temperatures and will find use in thermal engines, compressor stages of jet engines, coal gasification plants and the petrochemical industry.
A publication by J. H. Schneibel entitled xe2x80x9cSelected Properties of Iron Aluminidesxe2x80x9d, pp. 329-341, presented at the 1994 TMS Conference discloses properties of iron aluminides. This article reports properties such as melting temperatures, electrical resistivity, thermal conductivity, thermal expansion and mechanical properties of various FeAl compositions.
A publication by J. Baker entitled xe2x80x9cFlow and Fracture of FeAlxe2x80x9d, pp. 101-115, presented at the 1994 TMS Conference discloses an overview of the flow and fracture of the B2 compound FeAl. This article states that prior heat treatments strongly affect the mechanical properties of FeAl and that higher cooling rates after elevated temperature annealing provide higher room temperature yield strength and hardness but lower ductility due to excess vacancies. With respect to such vacancies, the articles indicates that the presence of solute atoms tends to mitigate the retained vacancy effect and long term annealing can be used to remove excess vacancies.
A publication by D. J. Alexander entitled xe2x80x9cImpact Behavior of FeAl Alloy FA-350xe2x80x9d, pp. 193-202, presented at the 1994 TMS Conference discloses impact and tensile properties of iron aluminide alloy FA-350. The FA-350 alloy includes, in atomic %, 35.8% Al, 0.2% Mo, 0.05% Zr and 0.13% C.
A publication by C. H. Kong entitled xe2x80x9cThe Effect of Ternary Additions on the Vacancy Hardening and Defect Structure of FeAlxe2x80x9d, pp. 231-239, presented at the 1994 TMS Conference discloses the effect of ternary alloying additions on FeAl alloys. This article states that the B2 structured compound FeAl exhibits low room temperature ductility and unacceptably low high temperature strength above 500xc2x0 C. The article states that room temperature brittleness is caused by retention of a high concentration of vacancies following high temperature heat treatments. The article discusses the effects of various ternary alloying additions such as Cu, Ni, Co, Mn, Cr, V and Ti as well as high temperature annealing and subsequent low temperature vacancy-relieving heat treatment.
The invention provides an aluminum-containing iron-based alloy useful as an electrical resistance heating element. The alloy has improved room temperature ductility, resistance to thermal oxidation, cyclic fatigue resistance, electrical resistivity, low and high temperature strength and/or high temperature sag resistance. In addition, the alloy preferably has low thermal diffusivity.
The heating element according to the invention can comprise, in weight %, over 4% Al, xe2x89xa70.1% oxide dispersoid particles or xe2x89xa61% Cr and  greater than 0.05% Zr or ZrO2 stringers oriented perpendicular to an exposed surface of the heating element. The alloy can comprise, in weight %, 14-32% Al, xe2x89xa62.0% Ti, xe2x89xa62.0% Si, xe2x89xa630% Ni, xe2x89xa60.5% Y, xe2x89xa61% Nb, xe2x89xa61% Ta, xe2x89xa610% Cr, xe2x89xa62.0% Mo, xe2x89xa61% Zr, xe2x89xa61% C, xe2x89xa60.1% B, xe2x89xa630% oxide dispersoid, xe2x89xa61% rare earth metal, xe2x89xa61% oxygen, xe2x89xa63% Cu, balance Fe.
According to various preferred aspects of the invention, the alloy can be Cr-free, Mn-free, Si-free, and/or Ni-free. The alloy preferably has an entirely ferritic austenite-free microstructure which optionally may contain electrically insulating and/or electrically conductive ceramic particles such as Al2O3, Y2O3, SiC, SiN, AlN, etc. Preferred alloys include 20.0-31.0% Al, 0.05-.0.15% Zr, xe2x89xa60.1% B and 0.01-0.1% C; 14.0-20.0% Al, 0.3-1.5% Mo, 0.05-1.0% Zr and xe2x89xa60.1% C, xe2x89xa60.1% B and xe2x89xa62.0% Ti; and 20.0-31.0% Al, 0.3-0.5% Mo, 0.05-0.3% Zr, 0.1% C, 0.1% B and xe2x89xa60.5% Y.
The electrical resistance heating element can be used for products such as heaters, toasters, igniters, heating elements in electrical cigarette smoling system, etc. wherein the alloy has a room temperature resistivity of 80-400xcexc xcexa9xc2x7cm, preferably 90-200xcexc xcexa9xc2x7cm. The alloy preferably heats to 900xc2x0 C. in less than 1 second when a voltage up to 10 volts and up to 6 amps is passed through the alloy. When heated in air to 1000xc2x0 C. for three hours, the alloy preferably exhibits a weight gain of less than 4%, more preferably less than 2%. The alloy can have a contact resistance of less than 0.05 ohms and a total heating resistance in the range of 0.5 to 7, preferably 0.6 to 4 ohms throughout a heating cycle between ambient and 900xc2x0 C. The alloy preferably exhibits thermal fatigue resistance of over 10,000 cycles without breaking when pulse heated from room temperature to 1000xc2x0 C. for 0.5 to 5 seconds.
With respect to mechanical properties, the alloy has a high strength to weight ratio (i.e., high specific strength) and should exhibit a room temperature ductility of at least 3%. For instance, the alloy can exhibit a room temperature reduction in area of at least 14%, and a room temperature elongation of at least 15%. The alloy preferably exhibits a room temperature yield strength of at least 50 ksi and a room temperature tensile strength of at least 80 ksi. With respect to high temperature properties, the alloy preferably exhibits a high temperature reduction in area at 800xc2x0 C. of at least 30%, a high temperature elongation at 800xc2x0 C. of at least 30%, a high temperature yield strength at 800xc2x0 C. of at least 7 ksi, and a high temperature tensile strength at 800xc2x0 C. of at least 10 ksi.
According to one aspect of the invention, an electrical resistance heating element formed from an iron aluminide alloy includes, in weight percent, over 4% Al and Zr in an amount effective to form zirconium oxide stringers perpendicular to an exposed surface of the heating element and pin surface oxide on the heating element during temperature cycling between ambient and temperatures over 500xc2x0 C.
According to another aspect of the invention, an electrical resistance heating element of an iron based alloy includes, in weight percent, over 4% Al and at least 0.1% oxide dispersoid, the oxide being present as discrete oxide dispersoid particles having sizes such as 0.01 to 0.1 xcexcm in a total amount of up to 30% and the dispersoid particles comprising oxides such as Al2O3 and Y2O3.
The invention also provides a process of making an alloy suitable for an electrical resistance heating element. The process includes forming an oxide coated powder by water atomizing an aluminum-containing iron-based alloy and forming powder having an oxide coating thereon, forming a mass of the powder into a body, and deforming the body sufficiently to break up the oxide coating into oxide particles and distribute the oxide particles as stringers in a plastically deformed body. According to various aspects of the method, the body can be formed by placing the powder in a metal can and sealing the metal can with the powder therein. Alternatively, the body can be formed by mixing the powder with a binder and forming a powder mixture. The deforming step can be carried out by hot extruding the metal can and forming an extrusion or extruding the powder mixture and forming an extrusion. The extrusion can be rolled and/or sintered. The iron-based alloy can be a binary alloy and the powder can contain in excess of 0.1 wt % oxygen. For instance, the oxygen content can be 0.2-5%, preferably 0.3-0.8%. In order to provide an electrical resistance heating element which heats to 900xc2x0 C. in less than one second when a voltage of up to 10 volts and up to 6 amps is passed through the alloy, the plastically deformed body preferably has a room temperature resistivity of 80-400xcexc xcexa9xc2x7cm. Due to the water atomizing of the powder, the powder is irregular in shape and the oxide particles consist essentially of Al2O3. The powder can have any suitable particle size such as 5-30 xcexcm.
The electric resistance heating material can be prepared in various ways. For instance, the raw ingredients can be mixed with a sintering additive prior to thermomechanically working the material such as by extrusion. The material can be prepared by mixing elements which react during the sintering step to form insulating and/or electrically conductive nietal compounds. For instance, the raw ingredients can include elements such as Mo, C and Si, the Mo, C and Si forming MoSi2 and SiC during the sintering step. The material can be prepared by mechanical alloying and/or mixing preal.loyed powder comprising pure metals or compounds of Fe, Al, alloying elements and/or carbides, nitrides, borides, silicides and/or oxides of metallic elements such as elements from groups IVb, Vb and VIb of the periodic table. The carbides can include carbides of Zr, Ta, Ti, Si, B, etc., the borides can include borides of Zr, Ta, Ti, Mo, etc., the silicides can include silicides of Mg, Ca, Ti, V, Cr, Mn, Zr, Nb, Mo, Ta, W, etc., the nitrides can include nitrides of Al, Si, Ti, Zr, etc., and the oxides can include oxides of Y, Al, Si, Ti, Zr, etc. In the case where the FeAl alloy is oxide dispersion strengthened, the oxides can be added to the powder mixture or formed in situ by adding pure metal such as Y to a molten metal bath whereby the Y can be oxidized in the molten bath, during atomization of the molten metal into powder and/or by subsequent treatment of the powder.
The invention also provides a powder metallurgical process of ma.king an electrical resistance heating element by atomizing an aluminum-containing iron-based alloy, forming a mass of the powder into a body, and deforming the body into an electrical resistance heating element. The body can be formed by placing the powder in a metal can, sealing the metal can with the powder therein followed by subjecting the can to hot isostatic pressing. The body can also be formed by slip casting wherein the powder is mixed with a binder and formed into a powder mixture. The deforming step can be carried out in various manners such as by cold isostatic pressing or extruding the body. The process can further include rolling the body and sintering the powder in an inert gas atmosphere, preferably a hydrogen atmosphere. If the powder is pressed, the powder is preferably pressed to a density of at least 80% so as to provide a porosity of no greater than 20% by volume, preferably a density of at least 95% and a porosity of no greater than 5%. The powder can have various shapes such as an irregular shape or spherical shape.